Light pulse analyzing detector system

ABSTRACT

In the detector system disclosed herein, a transient light signal, such as a modulated laser pulse, is analyzed or demodulated by generating, from the detected light, an electron beam which is linearly scanned across a phosphor screen. A linear array of photodetectors, responsive to the light emitted from corresponding portions of the screen, provide respective electrical signals having amplitudes corresponding to the instantaneous intensity of the light signal at respective times within the transient interval. These electrical signals are integrated and held in respective circuits for subsequent commutation and conversion to digital form.

455-605 AL) 233 E )(5 /2 W PIP-8105 xx 3,765,? 3,766,392 Nelson et :1.Jet. 16, 1973 Suas'mu F 7g I 1 LIGHT 3.675. 22 1/1972 Nelson et stmmwm250/199 SY Ra If 8 Zach:

Primary Examiner u e [75] humor! m 52 m Atromey-Ralph L. CadwalladerCaliti; Jerry G. Lackey, Charleston Heights, s.c. [571 ABSTRACT 1n thedetector system disclosed herein, a transient I73] Amgnee Ecac' Redfordlight signal, such as a modulated laser pulse, is anag [22] Filed: June11, 1971 lyzed or demodulated by generating, from the detected hght, anelectron beam whrch 1s lmearly [2]]. App scanned across a phosphorscreen A lmear array of photodetectors, responsive to the lxght emittedfrom IS2] US. Cl. 250/199 corresponclmg portions of the screen, pronderespec- [5l] Int. CL H041: 9/00 tive electrical signals havingamplitudes correspondmg [58] Field of Search 250/199, 213 R; to theinstantaneous intensity of the light signal at re- 313/75 specuve nmes\mhm the transient intenal These electrlcal signals are integrated andheld m respecuve (56] References Cited crrcuits for subsequentcommutauon and conversion UNITED STATES PATENTS w digital 3,569,6163/1971 Baker 250/199 X 10 Claims, 3 Drawing Figures SENSOR 2/ MODULATORREADOUT SUPPLY 1 29 CIRCUITS PATENTEUBCHSIHB 3.756.392

SENSOR 2/ l '23 MODULATOR #335: REAo uT suP lY /9 SYNC.

45 a N INTEGRATION AND HOLD UNEAR CIRCUITS 77 SYNC. DEFLECTION CIRCUITCOMMUTATOR 75 A-D CONVERTER F/G Z 79 DIGITAL M 67 STORAGE/PRINT-OUT 6563 7/ MELVIN A. NELSON --OUTPUT JOHN c. CLARK JERRY s. LACKEY 6.9 I ByFla 3 WPM LIGHT PULSE ANALYZING DETECTOR SYSTEM The invention describedherein was made in the course of work under contract with the US. AtomicEnergy Commission.

BACKGROUND OF THE INVENTION This invention relates to a transient lightsignal detecting system and more particularly to such a system which isuseful in electro-optical telemetry systems employing laser lightsources it has been proposed heretofore to employ laserbased telemetrysystems for communicating data from environments which are subject tointense nuclear or electromagnetic radiation fields, particularly whenthe data to be communicated involves characteristics of a transientphenomenon. e.g., an atomic explosion. One difficulty with such systemsis that the data must be abstracted from a transient light signal. e.g.,a modulated laser beam, which may be relatively weak or attenuated.

Heretofore, the transient light signal being received has typically beenapplied to a photodetector so as to obtain a corresponding electricalsignal, the resulting electrical signal being then displayed on anoscilloscope where it is photographed to provide a record which can besubsequently studied and analyzed. One such system is disclosed inco-pending application Ser. No. 39,345 filed May 21, 1970 now [5.5. Pat.No. 3,675,022 and entitled Electro-optical Telemetry Systems. While thegeneration of a photographic record provides a continuing record, thedelay and manipulation required for use of the photographic medium is adisadvantage in many situations.

Among the several objects of the present invention may be noted theprovision of a detection system for transient light signals; theprovision of such a system which provides an intensity history of thedetected transient light signal; the provision of such a system in whichthe data defining the intensity history is essentially immediatelyavailable; the provision of such a systern in which the data isavailable in digital form; the provision of such a system which ishighly sensitive; the provision of such a system which is accurate andwhich is of relatively simple and inexpensive construction. Otherobjects and features will be in part apparent and in part pointed outhereinafter.

SUMMARY OF THE INVENTION Briefly, apparatus according to the presentinvention operates to provide a plurality of continuing electricalsignals which represent the intensity of a transient light signal atrespective successive pains in time within the transient interval. Theoptical signal is received or detected by means of a photo-cathode whichprovides an electron signal proportional to the intensity of the lightsignal. This electron signal is formed into a beam which is then sweptlinearly across the phosphor screen during the transient interval. Bymeans of a linear array of photodetectors adjacent the screen, aplurality of signals are generated each of which corresponds to thelight emitted from a respective portion of the screen. Each of thephotodetectors is provided with a respective circuit means for holding,e.g., by integration, the signal generated by the photodetector for aperiod sub stsntially longer thmt the transient interval. Accordlngly. aplurality of continuing signals are generated having amplitudesrepresenting the intensity of the op- 2ticalsignalateorrespondingpointsintimewithinthe transient interval.

BRIEF DESCRIPTION OF THE DRAWINGS HO. 1 is a diagrammatic illustrationof an electrooptical telemetry system employing a transient light signaldetecting system of the present invention;

FIG. 2 is a schematic block diagram of the transient light signaldetecting system used in the system of FIG. 1; and

HO. 3 is a somewhat schematic circuit diagram of an individualphotodetector and associated holding circuit employed in the detectorsystem of FIG. 2.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to H6. 1, theoverall system illustrated there is arranged for the communication ofdata acquired from a transient phenomenon occurring in an environmentwhich may be subject to intense nuclear or electromagnetic radiation. Asensor 11 is indicated as the source of such data. in accordance withthe practice of the present invention, the data to be communicated isimpressed on an optical carrier by means of a modulator 13. Modulator 13may, for example, comprise a Kerr cell modulator of the type describedin the previously identified, co-pending application. Preferably, onlythe sensor 11, modulator l3 and the associated electronic and opticalcomponents are located within the severe environmental field, thegeneration and detection of the carrier radiation being accomplishedoutside the severe field.

ln the system illustrated in H6. 1, optical carrier radiation (light) isgenerated by means of a laser 15. Laser 15 is energized by means of apower supply 17 whose operation is preferably synchronized with thetransient event which is to be studied or analyzed, e.g. by means of asuitable sync signal provided at a terminal 19. While laser 15 isassumed to be operated in a solcalled pulsed socalled it should beunderstood that the transient phenomenon to be studied will typicallytake place entirely within a single laser pulse. Thus, the modulator 13will impress upon the comparatively long pulse, a higher frequencyintensity modulation which conveys the data to be communicated. Acontinuous wave laser of suitable power could also be employed.

The optical carrier energy generated by the laser 15 is directed throughthe modulator 13 by a suitable opti cal system, such as mirrors, lightpipes or a prism as indicated at 21. Likewise, the modulated opticalsignal emerging from the modulator 13 is directed back out of the severeenvironment held by a suitable optical system, indicated generally at23, where it is picked up and analyzed by a detector system inaccordance with.

the present invention, indicated generally at 25.

As illustrated in greater detail in FIG. 2, the modulated light pulse orsignal received back at the detector system is focussed on thephoto-cathode 31 of an image converter tube 33 by means of a suitableoptical systern, indicated generally at 35. As the light source isessentially collimated, due either to the nature of the laser or thedistance between the modulator and the detector, the received radiationis concentrated essentially at one point on the photo-athode. A suitablemask 37 may be employed to limit the field of view of this point on thephoto-cathode. As is conventional, image converter tube 33 includes anelectrostatic lens, indicated very diagrammatically at 39, for formingthe electrons emitted from the photo-cathode 31 into a beam or streamfocussed on the phosphor screen 41. in other words, the photo-cathode 31is imaged on the phosphor screen 41. The electrons are also acceleratedby a suitable potential gradient produced by an appropriate electrodestructure (not shown). As is also conventional, tube 33 includesdeflection or image position electrodes. The electrodes permittingdeflection on one axis are indicated at 43 and 45 in FIG. 2.

in the practice of the present invention, the image converter tube 33 isoperated in the so-called streak" mode, that is, the beam of ele-tronsformed from the electrons emitted by the photo-cathode 31, is sweptacross the phosphor screen 41 as a function of time. In FIG. 2, asuitable linear deflection circuit for providing suita JlB deflectionvoltages to the electrodes 43 and 45 is indicated at 47. Thesynchronization signal which controls the generation of the laser pulseis also applied to the deflection circuit 47 so that the scanning of theelectron beam in the image conversion tube is suitably synchronized withthe transmitted and received optical pulses.

Mounted closely adjacent the phosphor screen 41 is a linear array ofphotodetectors SlA-SlN. The axis of the linear array is aligned with thedirection of beam sweep produced by the deflection electrodes 43 and 45and the array is positioned so that the photodetectors SlA-SlN respondto the streak of light produced by the sweep of the electron beam acrossphosphor screen 41. in the embodiment illustrated in FIG. 2, thephotodetectors 51A-51N are assumed to be in such close proximity to thephosphor screen 41 that each photodetector is responsive to the lightgenerated by a corresponding portion of the screen. However, it shouldbe understood that the screen could be imaged upon the detector array bymeans of a suitable optical system so as to provide the desiredcorrespondence of individual detectors with respective portions of thescreen 41 and that the detectors should likewise be considered to be"adjacent" the screen in such an arrangement.

Each of the photodetectors 51A-51N is interconnected with andessentially incorporated into a respective integration and hold circuit,tnese circuits being indicated generally at 55. Each of the respectiveholding or integration circuits operates to provide a continuingelectrical signal which is essentially proportional to the integratedvalue of light received by the respective photodetector during a givensweep or streak of the electron beam in the image converter tube 33.

Assuming that the photodetectors 51A-51N are photodiodes, a suitableintegration and holding circuit is illustrated in FIG. 3. Thephotodetector diode 61 is connected to the input of an integratingamplifier 63, e.g., through a resistance 65, and a charge-storagecapacitor 67 is connected between the output and input of the amplifier.As will be apparent to those skilled in the art, this is essentially aconventional sample-and= hold circuit with the photo-detector acting assignal source and sampling gate. The sync signal is employed to resetall of the integration and holding circuits 55 at the start of a sweepor streak cycle and, in F16. 3, the resetting circuit is represented ascomprising'merely a simple switch 69 shunting the charge-storagecapacitor 67. However, as will also be understood by those skilled inthe art, this switch will. in practice, he preferably constituted by anelectronic switching circuit such as a tieldefiect transistor. Thecircuit of FIG. 3 operates to provide, at an output terminal 71, anelectrical signal having an amplitude which is proportional to theamount of light received by the photodiode 61, which signal voltagecontinues or persists substantially longer than the sweep interval,i.e., the persistence is limited only by the leakage of the capacitor 67and the bias current and stability characteristics of the amplifier 63.

The output signals provided by the several integration and hold circuits55 are applied in sequence to an analog-to-digital converter by means ofa commutator 77. The timed sequence of operation of the comm utator ispreferably initiated by the same sync signal which initiates the beamsweep and the resetting of the integration and hold circuits. However,the continuing nature of the multiple output signals provided by theholding circuits 55 allows this sequencing and conversion to take placeover a relatively long interval as compared with the transient event andstreak period which may be in the order of 10 microseconds. As thesevarious amplitude samples are successively converted to digital form,they may be printed out, e.g., on paper or magnetic tape, or enteredinto storage in conventional manner. As the amplitude samples areavailable on a continuing bais, that is, on a time scale which is longin comparison with the original transient optical signal, they may bereadily converted to digital form and entered into storage at a speedwhich is compatible with the requirements of the core memories typicallyassociated with digital computers. The storage and/or printing out may,of course, be coordinated with the opera tion of the commutator 77 inconventional manner to facilitate identification and correlation of theseveral samples.

As may be seen, the resolution of the system, i.e. the number of sampleswhich can be obtained for a given transient optical signal, is limitedsubstantially only by the optical resolution of the image conversiontube and the size of the individual photodetectors 51A-51N which make upthe linear array. Since the array comprises only a linear arrangement ofindividual photodetectors, rather than a two-dimensional matrix, it canbe seen that the array can be relatively easily fabricated usingconventional integrated circuit techniques to obtain a relatively largenumber of individual detectors for a given streak length. Thus, highresolution can be obtained.

While the optical carrier radiation utilized has been referred to hereinas light, this term should be interpreted to include infra-red andultraviolet radiation as well as visible light. Likewise, whilephotodiodes have been shown and described by way of illustration, it

should be understood that other types of photodetectors, such as PINdiodes or photo-FETs (photo fieldeflect transistors), may alsr be used.in the ease of the photo field-efiect transistors, an additionaladvantage obtainable is that the gain of the transistor itself may beutilized in the integration and hold circuitry so that the overallconstruction is In view of the foregoing, it may be seen that seven]objects of the present invention are achieved and other advantageousresults have been attained.

S Asvariouschangescouldbemadeintheaboveconstructions without departingfrom the scope of the invention. it should be understood that all mattercontainedintheabovedescriptionorshownintheaccompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

What is claimed is: 1. Apparatus for providing a plurality of continuingelectrical signals representing the intensity of a transient lightsignal at respective successive points in time within the transientinterval, said apparatus comprising:

a photo-cathode for receiving said optical signal and providing anelectron signal proportional thereto;

a phosphor screen adapted to receive impinging electrons and to emitlight in proportion to the intensity of the impinging electrons;

means for directing towards said screen a stream of electrons having anintensity proportional to said electron signal;

a linear array of photodetectors adjacent said screen,

each photodetector being responsive to light emitted from acorresponding portion of said screen, each of said photodetectors beingprovided with circuit means for holding a signal proportional to thesignal generated by the respective photodetector for a periodsubstantially longer than said transient interval; and

means for sweeping said stream of electrons across the portion of saidscreen adjacent said array as a function of time during said transientinterval thereby to generate in each of said circuit means a respectiveheld electrical signal having an amplitude corresponding to theintensity of the optical signal at a corresponding point in time withinthe transient interval.

2. Apparatus for providing digital data representing the intensityhistory of a transient light signal, said apparatus comprising:

a photo-cathode for receiving said optical signal and providing anelectron signal essentially proportional thereto;

a phosphor screen adapted to receive impinging electrons and to emitlight in proportion to the intensity of the impinging electrons;

means for directing towards said screen a stream of electrons having anintensity proportional to said electron signal;

a linear array of photodetectors adjacent said screen.

each photodetector being responsive to light emitted from acorresponding portion of said screen;

a plurality of integrator circuits, each incorporating a respective oneof said photodetectors and operating to provide a continuing signalhaving an amplitude which is a function of the light energy received bythe respective photodetector,

means for sweeping said stream of electrons across the portion ofsaidscreen seen by said array as a function of time during said transientinterval thereby to generate in each of said integrator circuits arespective, held electrical signal having an amplitude corresponding tothe intensity of said 6 representing the intensity history of said lightsignal.

3. Apparatus as set forth in claim 2 wherein said photodetectorscomprise photodiodes.

4. Apparatus as set forth in claim 2 wherein said photodetectorscomprise photo field-effect transistors.

5. Apparatus as set forth in claim 2 wherein said linear array ofphotodetectors comprises a plurality of detectors within a singleintegrated circuit.

6. Apparatus as set forth in claim 2 wherein said photocathode, saidphosphor screen. and said electron stream forming means compriseportions of an image converter tube.

7. Apparatus as set forth in claim 2 wherein said integrator circuitseach comprise an amplifier and a charge storage capacitor connecting theoutput of the amplifier to its input.

8. An electro-optical tele n etr systern for communicating dataconcerning a transient phenomenon acquired by a sensoryt'hich is remotefrom a desired data acquisition location, said system comprising:

an optical modulator interconnected with said sensor for modulatingincident light as a function of data, concerning said transientphenomenon, acquired by said sensor;

a laser provided with means for energizing said laser to produce lightduring the transient intervale, said light being directed through saidmodulator;

an image converter tube disposed to receive the light directed throughsaid moudlator and having a photo-cathode, a phosphor screen, and meansfor accelerating electrons emitted from said cathode and focussing theresultant electron stream on said screen;

means for operating said image converter tube in the streak mode inwhich said electron stream is swept essentially linearly across saidscreen thereby producing a light streak having a brightness at eachpoint along its length corresponding to the intensity of the electronstream at a corresponding point in time within the streak period;

means for synchronizing the image converter tube sweep with theoperation of said laser;

a linear array of photodetectors, each photodetector being responsive tolight emitted from a respective portion of said screen covered by saidstreak; and

a plurality of integrator circuits, each incorporating a respective oneof said photodetectors and operat ing to provide a continuing signalhaving an amplitude which is a function of the light energy received bythe respective photodetector, whereby the amplitude of each such signalcorresponds to the intensity of the modulated laser signal at acorresponding point in time within the transient interval.

9. A system as set forth in claim 8 wherein said modulight signal at acorresponding point in time within acquisition location, said systemcomprising:

an optical modulator responsive to said sensor for modulating incidentlight as a function of data, concerning said transient phenomenon,acquired by said sensor;

alaserprovided withmeansl'orenergizingsaidlaser to produce light duringthe transient interval. said light being directed through saidmodulator;

a photo-cathode for receiving laser light, modulated by said modulator.at a location remote from said sensor and providing an electron signalessentially proportional to that light;

a phosphor screen adapted to receive impinging electrons and to emitlight in proportion to the intensity of the impinging electrons;

means for directing towards said screen a stream of electrons having anintensity proportional to said electron signal;

a linear array of photodetectors adjacent said screen,

each photodetector being responsive to light emitted from acorresponding portion of said screen;

a plurality of integrator circuits. each incorporating a respective oneof said photodetectors and operating to provide a continuing signalhaving an amplitude which is a function of the light energy received bythe respective photodetector;

means for sweeping said stream of electrons across the portion of saidscreen seen by said array as a function of time during said transientinterval thereby to generate in each of said integrator circuits arespective, held electrical signal having an amplitude corresponding tothe intensity of the modulated laser light at a corresponding point intime within the transient interval;

an analog to digital converter; and

means for applying said held signals in sequence to said converterthereby to provide in digital form data obtained by said sensor.

Q C I Q 0

1. Apparatus for providing a plurality of continuing electrical signalsrepresenting the intensity of a transient light signal at respectivesuccessive points in time within the transient interval, said apparatuscomprising: a photo-cathode for receiving said optical signal andproviding an electron signal proportional thereto; a phosphor screenadapted to receive impinging electrons and to emit light in proportionto the intensity of the impinging electrons; means for directing towardssaid screen a stream of electrons having an intensity proportional tosaid electron signal; a linear array of photodetectors adjacent saidscreen, each photodetector being responsive to light emitted from acorresponding portion of said screen, each of said photodetectors beingprovided with circuit means for holding a signal proportional to thesignal generated by the respective photodetector for a periodsubstantially longer than said transient interval; and means forsweeping said stream of electrons across the portion of said screenadjacent said array as a function of time during said transient intervalthereby to generate in each of said circuit means a respective heldelectrical signal having an amplitude corresponding to the intensity ofthe optical signal at a corresponding point in time within the transientinterval.
 2. Apparatus for providing digital data representing theintensity history of a transient light signal, said apparatuscomprising: a photo-cathode for receiving said optical signal andproviding an electron signal essentially proportional thereto; aphosphor screen adapted to receive impinging electrons and to emit lightin proportion to the intensity of the impinging electrons; means fordirecting towards said screen a stream of electrons having an intensityproportional to said electron signal; a linear array of photodetectorsadjacent said screen, each photodetector being responsive to lightemitted from a corresponding portion of said screen; a plurality ofintegrator circuits, each incorporating a respective one of saidphotodetectors and operating to provide a continuing signal having anamplitude which is a function of the light energy received by therespective photodetector; means for sweeping said stream of electronsacross the portion of said screen seen by said array as a function oftime during said transient interval thereby to generate in each of saidintegrator circuits a respective, held electrical signal having anamplitude corresponding to the intensity of said light signal at acorresponding point in time within the transient interval; an analog todigital converter; and means for applying said held signals in sequenceto said converter thereby to provide digital data representing theintensity history of said light signal.
 3. Apparatus as set forth inclaim 2 wherein said photodetectors comprise photodiodes.
 4. Apparatusas set forth in claim 2 wherein said photodetectors comprise photofield-effect transistors.
 5. Apparatus as set forth in claim 2 whereinsaid linear array of photodetectors comprises a plurality of detectorswithin a single integrated circuit.
 6. Apparatus as set forth in claim 2wherein said photocathode, said phosphor screen, and said electronstream forming means comprise portions of an image converter tube. 7.Apparatus as set forth in claim 2 wherein said integrator circuits eachcomprise an amplifier and a charge storage capacitor connecting theoutput of the amplifier to its input.
 8. An electro-optical telemetrysystem for communicating data concerning a transient phenomenon acquiredby a sensor which is remote from a desired data acquisition location,said system comprising: an optical modulator interconnected with saidsensor for modulating incident light as a function of data, concerningsaid transient phenomenon, acquired by said sensor; a laser providedwith means for energizing said laser to produce light during thetransient intervale, said light being directed through said modulator;an image converter tube disposed to receive the light directed throughsaid modulator and having a photo-cathode, a phosphor screen, and meansfor accelerating electrons emitted from said cathode and focussing theresultant electron stream on said screen; means for operating said imageconverter tube in the streak mode in which said electron stream is sweptessentially linearly across said screen thereby producing a light streakhaving a brightness at each point along its length corresponding to theintensity of the electron stream at a corresponding point in time withinthe streak period; means for synchronizing the image converter tubesweep with the operation of said laser; a linear array ofphotodetectors, each photodetector being responsive to light emittedfrom a respective portion of said screen covered by said streak; and aplurality of integrator circuits, each incorporating a respective one ofsaid photodetectors and operating to provide a continuing signal havingan amplitude which is a function of the light energy received by therespective photodetector, whereby the amplitude of each such signalcorresponds to the intensity of the modulated laser signal at acorresponding point in time within the transient interval.
 9. A systemas set forth in claim 8 wherein said modulator comprises a Kerr cell.10. An electro-optical telemetry system for communicating dataconcerning a transient phenomenon acquired by a sensor which is remotefrom a desired data acquisition location, said system comprising: anoptical modulator responsive to said sensor for modulating incidentlight as a function of data, concerning said transient phenomenon,acquired by said sensor; a laser provided with means for energizing saidlaser to produce light during the transient interval, said light beingdirected through said modulator; a photo-cathode for receiving laserlight, modulated by said modulator, at a location remote from saidsensor and providing an electron signal essentially proportional to thatlight; a phosphor screen adapted to receive impinging electrons and toemit light in proportion to the intensity of the impinging electrons;means for directing towards said screen a stream of electrons having anintensity proportional to said electron signal; a linear array ofphotodetectors adjacent said screen, each photodetector being responsiveto light emitted from a corresponding portion of said screen; aplurality of integrator circuits, each incorporating a respective one ofsaid photodetectors and operating to provide a continuing signal havingan amplitude which is a function of the light energy received by therespective photodetector; means for sweeping said stream of electronsacross the portion of said screen seen by said array as a function oftime during said transient interval thereby to generate in each of saidintegrator circuits a respective, held electrical signal having anamplitude corresponding to the intensity of the modulated laser light ata corresponding point in time within the transient interval; an analogto digital converter; and means for applying said held signals insequence to said converter thereby to provide in digital form dataobtained by said sensor.